There has been known a battery system including a secondary battery and a bypass circuit that is provided with a semiconductor switch element and is connected parallely to the secondary battery. Such a battery system causes excessive charge current to flow to the bypass circuit so as to suppress overcharge to the secondary battery, and causes the secondary battery having been charged excessively and unexpectedly to discharge to the bypass circuit so as to reduce voltage of the secondary battery. An assembled battery, which includes a plurality of connected secondary batteries as described in Patent Document 1 listed below, suppresses charging variation between the secondary batteries.
This battery system can optionally include a resistor in the bypass circuit, so as to heat the secondary battery by means of heat generated by the resistor. The semiconductor switch element in the bypass circuit does not generate so much heat. The semiconductor switch element has been conventionally provided with a small heat sink in order to radiate heat generated by the semiconductor switch element.